Enhancing the phenolic profile and antioxidant activity in corn silk via solid-state fermentation with Aspergillus niger

Corn silk (Stigma maydis), an agricultural residue of maize cultivation, is rich in bioactive compounds, being however underutilized. This study aimed to evaluate whether solid-state fermentation (SsF) with Aspergillus niger can be applied as a sustainable bioprocessing strategy to enhance the phenolic profile and antioxidant potential of corn silk. Throughout 10 days of fermentation, a notable increase in total phenolic content (73 % increase via the Folin-Ciocalteu method) and total flavonoid content (37 % increase via the AlCl₃ method) was observed. HPLC-DAD-ESI-MS analysis identified 17 individual phenolic compounds, primarily flavones and hydroxycinnamic acids. Significant increases were recorded for 4-caffeoylquinic, 5-synapoylquinic, 4-feruloylquinic, and 3-synapoylquinic acids, indicating effective enzymatic release of bound phenolics by A. niger, while levels of flavone glycosides declined and their corresponding aglycones increased. Simultaneously, antioxidant activity also improved significantly in ABTS⁺, DPPH, and FRAP assays, though no substantial increase was detected using the CUPRAC method. Additionally, A. niger metabolized available saccharides (glucose, fructose, and xylose) and produced organic acids, including oxalic, citric, and malic acids, as fermentation by-products. Overall, this study demonstrates that SsF with A. niger is a promising approach for improving the bioactive composition of corn silk, supporting its valorization as a low-cost substrate for the development of functional foods, nutraceuticals, or other value-added applications within the circular bioeconomy.